It is sometimes necessary to test manufactured items for very minute leaks through portions of the item defining at least in part, a fluid tight cavity, such as the walls of fuel tanks and the rims of automotive wheels.
If such testing is done on every item, or a substantial proportion of the items produced, high volume production rate testing is necessary, with only a minimum test interval desirable. When testing for very minute leaks, achieving reliable detection in a short test interval is difficult.
A method heretofore employed is tract gas leak detection, in which the item is placed in a test fixture and a pressure differential applied across the portions of the item defining the fluid cavity, so that a trace gas containing fluid will be caused to pass through any leaks. Trace gas containing fluid passing through the item portion is collected, and passed over a trace gas detector of a type able to detect very minute concentrations of trace gas, with any increase in trace gas concentrations during the time the item is subjected to the differential pressure measured by the detector to detect leakage through the item portions. Examples of such method are disclosed in U.S. Pat. No. 4,584,877 and copending application Ser. No. 873,518 filed on May 23, 1986.
Typically, leaks must be of a predetermined magnitude in order to require failure of a tested item, so that the magnitude of detected levels of trace gas concentrations are relied on to measure the size of any leaks. Typically, the detected trace gas concentration levels are compared with a preselected threshold level corresponding to leakage of an unacceptable size.
Since many such tests are usually conducted successively over extended periods of time with the same apparatus, a problem has occurred in maintaining reasonably precise correlation between the detected trace gas concentration and the size of the leaks in the tested item.
This problem typically is caused by increases in the ambient levels of trace gas in the vicinity of the trace gas detector, as some trace gas escapes incidental to each test and causes a concentration increase in the air about the area of the test. Despite efforts to purge the test apparatus and shield the sensor from stray trace gas sources, this increase in ambient levels causes the trace gas sensor to read out an indication not only of increased trace gas concentration due to test item leakage, but the increase in ambient levels thereby creating error in the test results.
Another cause of such degradation of correlation of test results is drift in the electronic circuiting used to process the trace gas detector output signal. The phenomenon is herein referred to as the "zero drift".
Since the leak detection method depends on reasonably precise correlation between the detected concentration level of trace gas and the leak, accuracy of the test is compromised by such zero drift.
The time demands of production testing render impractical the calibration of equipment to zero prior to each test, and the skill level necessary for performance of the testing would be increased if such calibration were necessary.
Accordingly, it is the object of the present invention to provide a method and apparatus to automatically and reliably compensate for zero drift in a trace gas detector used for successive leak tests.